Certain applications for Global Positioning System (GPS) receivers require the receiver to be placed a thousand feet or more from the antenna. One such application is the use of GPS timing receivers for time synchronization and frequency control in telephone networks. This application of GPS timing receivers may require the antenna to be placed atop a large office building and routing a coaxial cable through the building to the telephone network control room where the GPS receiver is located. The losses in standard low cost coaxial cable will be between 10 dB and 30 dB per 100 feet at 1.57542 GHz. It is well known that losses in coaxial cable increase with frequency. For example, RG58C coaxial cable is listed in Buchsbaum's Handbook of Practical Electronics as having 1.6 dB of loss per 100 feet at 10 MHz and 24 dB per 100 feet at 1 GHz. Coaxial cable losses have been dealt with in the past in at least three ways.
The first approach involves the placement of a Low Noise Amplifier (LNA) at the antenna with enough gain to overcome the cable losses. However at GPS frequencies, this approach is useful only for cable lengths up to about 200 feet.
The second approach utilizes Low Noise Block (LNB) down conversion to a lower frequency at the antenna and transferring the signal at a lower frequency over a coaxial cable to a receiver designed to accept the lower frequency. This approach is not suitable for GPS receivers which are designed to receive the 1.57542 GHz signal. Nor is it a cost-effective solution to redesign the GPS receiver to accept a lower frequency signal.
The third approach seeks to deal with coaxial cable losses in a third way by using a down-converter section at the antenna and an up-converter section at the receiver to convert the signal back to the original frequency received at the antenna as is described in U.S. Pat. No. 5,999,795, with common inventorship and assignment as the present invention, the disclosure of which is hereby incorporated in its entirety. Briefly summarized, this invention prevents the introduction of frequency errors, a reference signal is transmitted along the cable and is used in both the up-conversion and down-conversion processes. Thus, a received signal at a frequency too high to be transmitted along a length of cable without appreciable loss in amplitude is down-converted to a lower frequency. The down-conversion process comprises mixing the received signal with a local oscillator signal to produce an intermediate frequency. The intermediate frequency is transmitted down the length of cable. The intermediate frequency is selected to be much lower than the frequency of the received signal. Since signal attenuation along the cable decreases as frequency decreases, the lower frequency intermediate signal will experience lower loss than would the higher frequency received signal. At the opposite end of the cable, the intermediate signal is up-converted to a higher frequency output signal. The up-conversion process is accomplished by mixing the intermediate signal with a local oscillator signal. The local oscillator signals in both the up-converter and down-converter are derived from the same reference signal. For GPS applications, a GPS signal at a frequency of 1.57542 GHz is received by an antenna and enters a converter where it is subtracted from 1.6368 GHz to yield a 61.38 MHz intermediate frequency. The intermediate frequency is amplified and enters a diplexer. The diplexer is an arrangement of a two-way power-splitter and filters that isolate the 61.38 MHz intermediate frequency and the 16.368 MHz reference frequency, thus allowing the coaxial cable to transfer both signals simultaneously. The intermediate frequency arrives at the diplexer on the receiving end of the coaxial cable and is directed to a converter where it is mixed with the 1.6368 GHz local oscillator (LO) signal to reproduce the signal at 1.57542 GHz. The output of the converter is then filtered and attenuated to a signal level that is representative of a signal received by an active GPS antenna with a gain of 30 dB. The GPS receiver connected to the output of the up-converter sees the signal as if it were connected to a standard active GPS antenna. This is a workable solution, but is too complex to allow addition of other frequencies into the same cable or fiber.
In addition, Radio Frequency fiber optic links now in use assume the need for high dynamic range. The assumption underlying these devices is that general purpose link will experience signals that range over a dynamic range of 60 dB or more, thus implying a signal power range of from about 0 dBm to −60 dBm. Adherence to this assumption causes existing devices to be very expensive. The expense derives, in part, from costs associated with special low noise linear modulators and detectors.
Thus, a need still exists for an inexpensive method and apparatus for transmitting GPS signals at L1 (1575.42 MHz) and/or L2 (1227.6 MHz) frequencies, as well as any frequency between 800 MHz and 1800 MHz down a long length, e.g., up to 2000 feet or longer, of fiber optic cable to the GPS receiver.